With known membrane pumps of the general subject type, a leak replenishment device is customarily provided which employs a snuffle link in the form of a channel connecting the compression space and the reservoir space. This channel is ordinarily closed off by a spring-loaded snuffle valve which is controlled by the underpressure in the hydraulic system which drives the membrane. When the piston of the pump, in the course of its suction stroke, produces an excess underpressure in the hydraulic system, due to an insufficiency of hydraulic fluid, the snuffle valve responds to the excess underpressure and opens up the fluid link between the compression space and the reservoir space in the hydraulic system. Thus as a result of the sensing of the excess underpressure by the snuffle valve, the deficient amount of hydraulic fluid in the compression space is supplied, via the channel coming from the reservoir space.
By convention, "excess underpressure" is understood to mean an underpressure the magnitude of which (considered as an absolute value, not a negative value) is greater than is desirable. Likewise, an "underpressure peak" is defined as a peak in the absolute value of the underpressure.
In such replenishment of leaks, a problem arises in that peaks in the underpressure occur at the beginning of the suction stroke of the piston, leading to premature response of the snuffle valve. In high pressure membrane pumps, such marked underpressure peaks, so-called "Joukowsky impulses", tend to occur at the beginning of the suction phase, when the liquid column in the suction line is accelerated backward when the suction valve is opened. The resulting premature response of the snuffle valve causes overfilling of the compression space with hydraulic fluid, resulting in excessive flexure of and excessive stress on the membrane during the compression stroke.
To avoid such a premature response of the snuffle valve as a consequence of underpressure peaks, and thereby to avoid the resulting overfilling of the hydraulic compression space with hydraulic fluid, it is known to influence the leak replenishment by means of a so-called "membrane-position-dependent control". With such a solution, the membrane itself actuates a control valve, wherewith the slide element of a slide valve controlled by the membrane is moved in response to the membrane position when the piston is in the region of its end position, wherewith, with the membrane generally in its end position, the slide valve opens the link between the reservoir space and the compression space. Under this arrangement, leak replenishment is carried out only when the membrane reaches a prescribed limit position when the hydraulic pump piston is at the end of the suction stroke.
Further, a variant membrane apparatus is known which can also prevent premature leak replenishment. In this apparatus, a control system is provided which is also influenced by the given membrane position, which control system employs spring-loaded control rods. The control rods hold the leak replenishment valve by mechanical means in a normally closed position. When the membrane in the end position of the suction stroke, i.e. with the pump piston at the end of the piston's suction stroke, causes a movably guided perforated plate to move in a certain way against the force of springs, a translational movement of the control rods results, releasing the leak replenishment valve.
Known control systems of this general type governed by the membrane position have the disadvantage that they are of complex design; further, they must be precisely adjusted in order to function accurately under conditions of short membrane excursions of the type which occur particularly in the case of high pressure membrane pumps having metal membranes.
Accordingly, the underlying problem of the invention is to devise a membrane pump of the general type under consideration, whereby the described disadvantages are eliminated, wherewith in particular the pump is inexpensive to design and fabricate and prevents premature response of the snuffle valve, particularly during the period when the actuating underpressure peak is present, so that the snuffle valve will not be opened by such peaks at the beginning of the suction stroke.